embedded snapshot: unlock the cache object

It it locked when returning from vlc_object_wait/vlc_cond_wait

src/control/mediacontrol_audio_video.c

@@ -94,6 +94,7 @@ mediacontrol_snapshot( mediacontrol_Instance *self,
     vlc_object_release( p_vout );
 
     p_snapshot = ( snapshot_t* ) p_cache->p_private;
+    vlc_object_unlock( p_cache );
     vlc_object_release( p_cache );
     vlc_object_release( p_input );